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Sapphire gemstones. Sapphires are a non-red variety of the mineral corundum (aluminium oxide). It is the second hardest mineral after diamond, rated 9 out of 10 on the Mohs scale. Although blue is the most common colour, golden, white and even black are used as gemstones. Inclusions of the mineral rutile (titanium oxide, not seen) causes a six-pointed-star light effect (asterism) to form star sapphire.
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